Light-emitting diodes (LEDs) are rectifying semiconductor devices that convert electrical energy into electromagnetic radiation. They are typically inorganic solid-state devices that emit radiation in the visible region to the near infrared, i.e., in a region of about 400-1300 nm. Generally, an inorganic solid-state LED consists of elements from Groups IIB, IIIA, IVA, and VA of the Periodic Table of the Elements (i.e., Groups 12-15). For example, a red light-emitting diode is made of a gallium arsenide (GaAs) semiconductor, which has a direct band gap. These, however, are not easily or economically used in large-area displays. This is because of the difficulty in scaling the fabrication to a large matrix of GaAs pixels, while using very tightly controlled conditions. Systems based on polycrystalline zinc sulfide (ZnS) have also been developed, although low efficiencies and poor reliability have prevented large-scale production.
Many semiconductor devices can be based on organic molecules as well. Because of the high photoluminescence quantum yields common in organic molecular semiconductors, light emission through charge injection under a high applied field (electroluminescence) is possible. Thus, light-emitting diodes can be fabricated from organic molecules. Typically, vacuum sublimation is used in the fabrication of such diodes. Although the efficiencies and selection of emission color are very good for organic-based LEDs, there are problems associated with the long-term stability of the sublimed organic film against recrystallization and other structural changes.
One way to improve the structural stability of these organic layers is to use macromolecular organic materials. Macromolecular organic compounds containing conjugated systems, such as conjugated polymers, are a good choice in that they can, in principle, provide good charge transport. The most recent interest surrounding .pi.-conjugated polymer-based electroluminescence (EL) in organic semiconductors has been stimulated by the discovery that sublimed molecular films can show high quantum efficiencies of luminescence. Thus, it has been recognized that .pi.-conjugated polymers can be used for their electroluminescent properties in organic semiconductors. For example, intensely photoluminescent (PL) .pi.-conjugated polymers such as the poly(3-alkylthiophenes) (P3ATs), poly(.rho.-phenylenes) (PPPs), and poly(.rho.-phenylenevinylenes) (PPVs) can also electroluminesce, and thereby be incorporated into diodes.
The LEDs incorporating .pi.-conjugated polymers typically sandwich the conjugated polymers directly between high and low work function metals or other conductors, which act as hole and electron-injecting contacts, respectively. As used herein, a "hole," i.e., an electron hole, is a vacant electron energy state, which behaves as though it were a positively charged particle. In these diodes, the injected positive and negative charges move through the conjugated polymer under the influence of an applied electric field. The charges either annihilate one another to form a triplet or a singlet exciton, of which only the singlet may decay radiatively, or they pass through the conjugated polymer layer to the electrode of opposite charge.
One of the first LEDs reported as incorporating conjugated polymers consisted of a rectifying metal contact of an emissive layer of poly(.rho.-phenylenevinylene) (PPV) sandwiched between indium tin oxide (ITO), which acts as a hole-injecting electrode, and an electron-injecting layer of aluminum. See, for example, J. H. Burroughes et al., Nature, 347, 539 (1990), and A. R. Brown et al., Appl. Phys. Lett., 61, 2793 (1992). Another example of an LED that incorporates a conjugated diode is Ca/poly(2-methyl, 5-(2'-ethylhexoxy)-.rho.-phenylenevinylene)/(doped polyaniline). See G. Gustafsson et al., Nature, 357, 477 (1992). In other diodes, the injected charge carriers are confined to the light-emitting layer by a polymeric electron-conducting-hole-blocking layer.
Although such conjugated polymer-based EL diodes appear to be very promising for the development of low-cost, multicolored, large-area active flat displays, there are a number of remaining impediments to commercialization that need to be corrected. For example, many organic polymer-containing diodes have low EL efficiencies, have deficient color-control capabilities, are unstable in air, and/or have very short operating lifetimes, i.e., only about a few minutes to a few days.
For example, the photon/(injected electron) yield of the Al/PPV/ITO LED, discussed above, is only about 10.sup.-4. Furthermore, it is unstable in air. That is, it degrades both through a catastrophic short and steady decline in the emission at constant current. Replacing the aluminum with calcium, a lower work-function element, however, causes a dramatic improvement in the efficiency of the LED, enhancing the photon/(injected electron) yield of the LED to about 1%, which is comparable to the yield of commercial GaAs-based LEDs. This Ca/PPV/ITO LED is very unstable for reasons not entirely clear, but may include diffusion of the Ca metal through the polymer layer or oxidation of the Ca. Although the Ca/poly(2-methyl, 5-(2'-ethylhexoxy)-.rho.-phenylenevinylene)/polyaniline LED, discussed above, is more stable, it still typically degrades continuously after a few hours. Furthermore, it fails catastrophically after degrading for a few days, and its emission is limited to the red-orange band of the visible spectrum.
The foremost obstacle towards the development of commercial diodes is the instability and degradation of the LEDs. The nature of the degradation, however, is not clear.